The signal level of black color of video signals transmitted to television receiving sets for the purpose of displaying images on the display screen of the television set can vary depending on the broadcasting station, the TV camera, the type of the home use VTR or the like. It is known that the apparent black contrast is improved by expanding (the amplitudes of) signals for black in the television receiving set that reproduces video signals. FIG. 1 of the accompanying drawings is a block diagram of a video signal-processing device 7 proposed in Japanese Patent Application Laid-Open Publication No. 7-154644, which is a typical black level expansion circuit.
The video signal-processing device 7 illustrated in FIG. 1 comprises an input terminal 71, a black expanding section 72, an output terminal 73, a black peak holding section 74, a comparator 75, a pedestal level transmitting section 76 and a gain control amplifier 77. The video signal-processing device 7 operates for feed back control in order to make the peak position of the input black level (to be referred to as black peak level hereinafter) agree with the pedestal level.
The black expanding section 72 performs an operation of black expansion on black level signals lower than a predetermined threshold level TH as shown in FIG. 2A out of the video signals input by way of the input terminal 71 depending on the feedback gain input from the comparator 75. The black expanding section 72 then transmits the black level signals that have been subjected to an operation of black expansion to the output terminal 73 and the black peak holding section 74. The video signals transmitted to the output terminal 73 are output to the television receiving set without modification.
The black peak holding section 74 sequentially takes out only the video signals that do not contain any synchronizing signal component out of the transmitted black level signals and detects the black peak level BL of each of the signals. The black peak holding section 74 applies the detected black peak level to the comparator 75. The configuration of the black peak holding section 74 will be described in greater detail hereinafter.
The comparator 75 receives the black peak level from the black peak holding section 74 and the pedestal level Ep as shown in FIG. 2B from the pedestal level transmitting section 76. Then, the comparator 75 determines the difference between the black peak level and the pedestal level Ep and outputs the difference to the gain control amplifier 77.
The gain control amplifier 77 generates a feedback gain on the basis of the black peak level and the pedestal level Ep input to it and transmits the feedback gain to the black expanding section 72. Note that the gain control amplifier 77 generates a feedback gain so as to reduce the difference between the black peak level and the pedestal level input to it. Upon receiving the feedback gain, the black expanding section 72 can perform an operation of black expansion so as to make the black peak level agree with the pedestal level Ep.
As the black expanding section 72 repeats the operation of transmitting a video signal that has been subjected to an operation of black expansion to the black peak holding section 74 for the black level signals, it is possible to make the black peak level gradually come close to the pedestal level Ep. Thus, the video signal-processing device 7 can converge only the video signals with a black level lower than the predetermined threshold level TH as shown in FIG. 2B to a constant luminance level and can hold the video signals other than black level signals without manipulating the amplitude so that it is possible to improve the black contrast, while maintaining the brightness of the entire image to a constant level.
Now, the black peak holding section 74 of the video signal-processing device 7 will be described in greater detail. FIG. 3 is a schematic circuit diagram of the black peak holding section 74. The black peak holding section 74 comprises an input section 81 for receiving video signals that have been subjected to an operation of black expansion from the black expanding section 72, a first transistor 82, a second transistor 83, a third transistor 84, a first power source 86, a second power source 87, a resistor 88, a black area control resistor 89, a capacitor 90, an output section 91 for applying the detected black peak level to the comparator 75, a first current source 92 and a second current source 93.
The first transistor 82 receives the video signal Be that is expanded for black from the input section 81 by way of its base terminal. Its emitter terminal is connected to the first current source 92, while its collector terminal is connected to the base terminal of the resistor 88 and the base terminal of the third transistor 84. The second transistor 83 has its base terminal connected to the second current source 93, the capacitor 90, the output section 91 and so on and is adapted to transmit the held black peak level to the comparator 75 by way of the output section 91. The third transistor 84 has its base terminal connected to the collector terminal of the first transistor 82 and the resistor 88 and its collector terminal is connected to the black area control resistor 89. Note that each of the first transistor 82, the second transistor 83 and the third transistor 84 is operated to become ON/OFF according to the potential difference between the base and the emitter thereof.
The resistor 88 has one of its ends connected to the collector terminal of the first transistor 82 and the base terminal of the third transistor 84 in order to regulate the voltage applied from the first transistor 82 to the third transistor 84. The black area control resistor 89 has one of its ends connected to the base terminal of the second transistor 83 and its other end connected to the collector terminal of the third transistor 84 in order to limit the electric current supplied from the second current source 93 and the capacitor 90 to the third transistor 84. The capacitor 90 typically stores an electric charge that corresponds to the current value of the black peak and is connected to the connection point of the above-described second transistor 83 and the black area control resistor 89 as well as to the output section 91 and the second current source 93.
Now, the operation of the black peak holding section 74 will be described below.
The first transistor 82 and the second transistor 83 form an operational amplifier for switching operations. The first transistor 82 becomes ON and the second transistor 83 becomes OFF when the base potential of the first transistor 82 is lower than the base potential of the second transistor 83, whereas the first transistor 82 becomes OFF and the second transistor 83 becomes ON when the base potential of the first transistor 82 is higher than the base potential of the second transistor 83.
When a video signal showing a black peak level lower than the currently held black peak level is input to the black peak holding section 74 having such an operational amplifier by way of the input section 81, the base potential of the first transistor 82 is lowered to make the first transistor 82 become ON. As a result, an electric current flows from the collector terminal of the first transistor 82 to the resistor 88 to raise the potential difference between the opposite ends of the resistor 88.
As the potential difference between the opposite ends of the resistor 88 rises to such an extent as to turn on the third transistor 84, the transistor 84 becomes ON and an electric current flows from the second current source 93 and the capacitor 90 to the third transistor by way of the collector terminal thereof. As a result, the base potential of the second transistor 83 is lowered. In other words, when a video signal whose black peak level is lower than the black peak level that is currently held by the black peak holding section 74 is input, the black peak holding section 74 lowers the black peak level it holds to the lower black peak level.
When, on the other hand, a video signal showing a black peak level higher than the currently held black peak level is input to the black peak holding section 74 by way of the input section 81, the base potential of the first transistor 82 is raised to make the second transistor 83 become ON. As a result, the first transistor 82 becomes OFF and hence the third transistor 84 also becomes OFF so that the electric current output from the second current source 93 is stored in the capacitor 90.
Thus, as the above sequence of operation is repeated, the lowest level of the input video signal, or the black peak level, is held to the base terminal of the second transistor 83.
FIG. 4 illustrates the relationship between the input video signal E and the black peak level BL held by the black peak holding section 74. In FIG. 4, dotted line Bp1 indicates the black peak level when the black area control resistor 89 is ignored. If point a11 in FIG. 4 shows the voltage currently held for the black peak and an input signal having a lower value is entered, the black peak holding level falls to point a12 and a new black peak is held. If the voltage of the input signal rises from there, the third transistor 84 in FIG. 3 becomes OFF and the electric current from the second current source 93 is stored in the capacitor 90. Since the current value from the second current source 93 is typically very small, the voltage of the output gradually rises. In this way, the black peak holding operation proceeds.
In FIG. 4, dotted line Bp2 indicates the black peak level when the black area control resistor 89 is made to show a large resistance. Since the electric current supplied to the third transistor 84 is limited by the black area control resistor 89, the capability of tracking the black peak holding level in response to the input signal falls so that the black peak holding level rises if compared with the dotted line Bp1 .
In other words, the black peak level that is held changes as a function of the resistance of the black area control resistor 89. For the black peak level indicated by the dotted line Bp2 falls to the black peak level indicated by the dotted line Bp1, the black area of the input signal needs to be greater. Differently stated, it is possible for the black peak holding section 74 of the video signal-processing device 7 to determine the area of a black region for which an operation of black expansion is conducted by means of the black area control resistor 89 by selecting the resistance of the black area control resistor 89.
With the above described known video signal-processing device 7, the black peak level held by the black peak holding section 74 theoretically undulates as shown by the dotted lines Bp1 and Bp2 in FIG. 4 so that, if black is expanded according to the black peak holding level, the image being displayed may show mainly vertical shading depending on the video signal, which is visually not pleasant to the viewers.
While the undulations of the black peak holding level can be reduced by increasing the capacitance of the capacitor 90 and/or reducing the electric current supplied from the second current source 93, there arises a problem that the known video signal-processing device 7 cannot respond sufficiently quickly when a video signal for a quickly changing scene is input.
Furthermore, when an analog system is used and the component blocks of the black peak holding section 74 shown in FIG. 3 are realized by using ICs, there arises a problem that the current values of the black area control resistor 89, the capacitor 90 and the first and second current sources 92, 93 can disperse to make it difficult for the video signal-processing device 7 to operate stably.